1. Field of the Invention
The present invention relates to a high frequency receiver for receiving a broadband high frequency signal.
2. Background Art
A conventional tuner will be described hereinafter with reference to a drawing. FIG. 26 is a schematic block diagram of the conventional tuner.
In FIG. 26, a high frequency signal of about 50 to 860 MHz is fed into input terminal 1.
Antenna filter 2 connected to input terminal 1 is a single tuning filter. Tuned frequency by antenna filter 2 is tuned to a frequency of a received channel, and an undesired frequency is removed. High-frequency amplifier 3 is connected to an output part of antenna filter 2 and amplifies the high frequency signal.
An output part of high frequency amplifier 3 is connected to step-to-step filter 4. Step-to-step filter 4 is a double tuning filter.
Mixer 5 is a double balanced mixer, and one input part thereof is connected to the output part of step-to-step filter 4 and the other input part is connected to the output part of frequency-variable local oscillator 6. Mixer 5 converts the frequency of a signal to a predetermined intermediate frequency (IF).
Band-pass filter 7 receives an output from mixer 5, and removes signals other than a desired signal. An output from band-pass filter 7 is supplied to output terminal 8.
Japanese Patent Examined Publication No. H7-297682, for example, is known as prior art information related to the present invention.
The frequency conversion by a mixer generally generates an image interfering signal, so that an image (this is simply called an image) for generating the image interfering signal must be previously suppressed with a filter on the upstream side of the mixer. The conventional high frequency receiver therefore increases a damping amount of the image with respect to frequency, using a filter having a high-performance damping characteristic in a two-stage structure of a single tuning filter and a double tuning filter.
The conventional high frequency receiver requires two filters, namely one double tuning filter having two tuning circuits and one single tuning filter, so that the receiver is expensive.